The present invention relates to the liquefaction of coal. During the conversion or liquefaction of coal which contains calcium, calcium carbonate scale normally forms on the surfaces of the coal liquefaction reactor, lines, auxiliary equipment and the like. The scale is extremely troublesome while attempting to obtain long continuous runs of the liquefaction process because eventually the flow through the narrowest sections of the plant is drastically impeded or closed. Therefore, it is essential that calcium and the like foulants be rendered innocuous if a successful coal conversion process is desired. The present invention proposes to solve the carbonate scale deposition problem by contacting coal with a sulfur oxide to form calcium sulfite, contacting the treated coal and resulting calcium sulfite with hydrogen sulfide to produce water soluble thiosulfate, liquefying the treated coal in a coal conversion zone and then contacting the effluent from the coal conversion zone with water to remove the water soluble thiosulfate.
U.S. Pat. No. 4,161,440 (Brunson), teaches a method to minimize the formation of scale during coal liquefaction by adding sulfur oxide with the coal prior to liquefaction to form a molecular species which deposits within the pores of the coal. This resulting molecular species is thermally stable and does not decompose at liquefaction conditions, and during liquefaction remains as particulate solids and thereby does not form or at least suppresses the formation of scale, or calcium carbonate deposits. The insoluble form of calcium remains within the liquefaction bottoms, or ash, and is conveniently disposed of, after liquefaction, with the liquefaction bottoms.
Although the process of U.S. Pat. No. 4,161,440 (Brunson) reduces the formation of scale in the coal liquefaction zone, the quantity of ash which must be ultimately separated from the desirable products of the liquefaction is increased. Separation of ash is a large and difficult task associated with the liquefaction of coal and any effort to minimize the quantity of ash to be separated is advantageous. My invention converts ash precursors in contradistinction to U.S. Pat. No. 4,161,440 (Brunson) to water soluble compounds which may easily be separated from the liquefaction zone effluent.